how genes work
DESCRIPTION
How Genes Work. Transcription. The information contained in DNA is stored in blocks called genes the genes code for proteins the proteins determine what a cell will be like The DNA stores this information safely in the nucleus where it never leaves - PowerPoint PPT PresentationTRANSCRIPT
How Genes Work
Transcription• The information contained in DNA is stored in
blocks called genes the genes code for proteins the proteins determine what a cell will be like
• The DNA stores this information safely in the nucleus where it never leaves instructions are copied from the DNA into messages
comprised of RNA these messages are sent out into the cell to direct the
assembly of proteins
Transcription• The path of information is often referred to as the central
dogma
DNA RNA protein
• The use of information in DNA to direct the production of particular proteins is called gene expression, which takes place in two stages
transcription is the process when a messenger RNA (mRNA) is made from a gene within the DNA
translation is the process of using the mRNA to direct the production of a protein
Transcription
• RNA is the same as DNA except that the sugars in RNA have an extra oxygen and T is replaced by another pyrimidine called uracil (U)
• The cell uses three kinds of RNA messenger RNA (mRNA), ribosomal RNA
(rRNA), and transfer RNA (tRNA)
Transcription
• A protein called RNA polymerase produces the mRNA copy of DNA during transcription it first binds to one strand of the DNA at a site
called the promoter and then moves down the DNA molecule and assembles a complementary copy of RNA
transcription ends when the RNA polymerase reaches a certain nucleotide sequence that signals it stop
Transcription
• View “mRNA Synthesis (Translation)” –animation in my Website
Translation
• To correctly read a gene, a cell must translate the information encoded in the DNA into the language of proteins translation follows rules set out by the genetic
code the mRNA is “read” in three-nucleotide units
called codons• each codon corresponds to a particular amino acid
Translation
• The genetic code dictionary was determined from trial-and-error experiments to work out which codons matched with which amino acids
• The genetic code is universal and employed by all living things
The genetic code (RNA codons)1 start codon, 3 stop codons, and 60 codons
encode for 20 amino acids
There are 64 different codons in the genetic code.
Translation
• Translation occurs in ribosomes, which are the protein-making factories of the cell each ribosome is a complex of proteins and several
segments of ribosomal RNA (rRNA) ribosomes are comprised of two subunits
• small subunit • large subunit
the small subunit has a short sequence of its rRNA exposed that is identical to the leader sequence that begins all genes
• mRNA binds to the small subunit
Translation
• The large RNA subunit has three binding sites for transfer RNA (tRNA) located directly adjacent to the exposed rRNA sequence on the small subunit these binding sites are called the A, P, and E
sites it is the tRNA molecules that bring amino
acids to the ribosome to use in making proteins
A ribosome is composed of two subunits
Translation
• The structure of a tRNA molecule is important to its function it holds an amino acid attachment site at one
end and a three-nucleotide sequence at the other end
this three-nucleotide sequence is called the anticodon and is complementary to 1 of the 64 codons of the genetic code
If the codon for an amino acid is AUG, the anticodon is UAC.
The structure of tRNA.
Translation
• Once an mRNA molecule has bound to the small ribosomal subunit, the other larger ribosomal subunit binds as well, forming a complete ribosome during translation, the mRNA threads through
the ribosome three nucleotides at a time a new tRNA holding an amino acid to be
added enters the ribosome at the A site
Translation
• Before a new tRNA can be added, the previous tRNA in the A site shifts to the P site
• At the P site peptide bonds from between the incoming amino acid and the growing chain of amino acids
• The now empty tRNA in the P site eventually shifts to the E site where it is released
How translation works
Translation
• Translation continues until a “stop” codon is encountered that signals the end of the protein
• The ribosome then falls apart and the newly made protein is released into the cell
• View “How Translation Works” – animation in my Website.
Ribosomes guide the translation process
How protein synthesis works in eukaryotes
• View “Protein Synthesis” – animation in my Website